The present invention relates to the field of microelectronics and more particularly to dielectric films for microelectronic circuits.
A dynamic random access memory (DRAM) includes an array of memory cells with each memory cell having a capacitor and a transistor. As DRAMs become more highly integrated, the size of the capacitor and the operating voltage both decrease. Because a predetermined charge is required on the capacitor to discriminate between logic levels, reductions in the size of the capacitor may be limited by the capacitance required to store the predetermined charge. The electrical charge Q of a capacitor is determined by multiplying the capacitance C by the operating voltage V. Accordingly, in order to store a predetermined charge at a lowered operating voltage, the capacitance of the capacitor must be increased.
The capacitance of a capacitor can be increased by increasing the effective area of the capacitor, by increasing the dielectric constant of the dielectric layer, and by decreasing the thickness of the dielectric layer. Because increasing the effective area of a capacitor may require an increase in the size of the capacitor, and because decreasing the thickness of the dielectric layer may be limited by manufacturing constraints, these approaches to increasing capacitance may not be sufficient. The use of dielectric materials having higher dielectric constants may, however, provide increased capacitance while reducing the size of the capacitor without requiring a dielectric thickness which is unnecessarily difficult to produce. For example, Ta2O5 can be used to produce a dielectric film having a significantly higher dielectric constant than a dielectric film of the same thickness formed from silicon oxide SiO2. The film formed from Ta2O5 can have a dielectric constant on the order of 20 to 25. Accordingly, when using a Ta2O5 film as the dielectric layer of a capacitor, the surface areas of the capacitor electrodes can be reduced without reducing the capacitance and without significantly increasing manufacturing costs.
A flowchart for a method of forming a capacitor including a Ta2O5 dielectric film is illustrated in FIG. 1. A method for forming a capacitor includes the steps of forming a lower electrode of the capacitor S1, cleaning the lower electrode surface to remove a naturally occurring oxide film therefrom S2, rapid thermal processing S3, depositing the Ta2O5 film S4, annealing the Ta2O5 film with ultraviolet light and ozone (O3) S5, annealing the Ta2O5 film with oxygen (O2) S6, and forming an upper electrode for the capacitor S7.
The step of rapid thermal processing S3 eliminates an oxidation barrier generated at an interface between the lower electrode and the Ta2O5 film. The rapid thermal processing step may be provided as a succession of heat treatments. The step of annealing the Ta2O5 film with ultraviolet light and ozone (O3) S5 reduces oxygen vacancies in the Ta2O5 film. The step of oxygen (O2) annealing S6 reduces weak spots in the Ta2O5 film.
It is known, for example, to reduce leakage current of a Ta2O5 film by annealing the film. An annealing method is described in a publication by Shinriki et al. entitled xe2x80x9cUV-O3 DRY-O2: Two-Step Annealed Chemical Vapor-Deposited Ta2O5 Films For Storage Dielectrics Of 64-Mb DRAM""sxe2x80x9d, IEEE Transactions On Electron Devices, Vol. 38, No. 3, March 1991, pp. 455-462, the disclosure of which is hereby incorporated herein by reference.
Typically, a deposition system is used to deposit the Ta2O5 film, and separate systems are used to anneal and clean the structure. Accordingly, the Ta2O5 dielectric film may be exposed to air resulting in the adsorption of water and/or free carbon. Accordingly, exposure to air may cause deterioration of the dielectric layer and the adjacent electrodes. In addition, the loading and unloading of the production substrates from one system to the next may result in unnecessary heating and cooling between each step as well as unnecessary transfers of the material thus reducing the efficiency of the process and lowering the throughput.
It is therefor an object of the present invention to provide improved methods and systems for forming dielectric films.
It is another object of the present invention to provide methods and systems for forming dielectric films with improved dielectric characteristics.
It is still another object of the present invention to provide methods and systems for forming dielectric films while reducing exposure to the atmosphere.
These and other objects according to the present invention are provided by methods and systems for forming dielectric films including the steps of placing a substrate in a process chamber, depositing the dielectric film on the substrate in the process chamber, and annealing the dielectric film in the process chamber. By both depositing and annealing the dielectric film in the process chamber, the substrate and film can be isolated from the external environment between the steps of depositing and annealing the dielectric film. Accordingly, adsorption of moisture and free carbon from the air can be reduced thus improving the characteristics of the dielectric film.
In particular, the dielectric film may comprise a material with a high dielectric constant, such as Ta2O5. Ta2O5 has a dielectric constant on the order of 20-25 which is significantly higher than that of silicon dioxide. Alternately, the film may comprise another material having a high dielectric constant, such as BaSrTiO3 (BST) or PbZrTiO3 (PZT).
In addition, the step of depositing the dielectric film can include raising the temperature inside the process chamber to a first temperature and depositing the dielectric film. The step of annealing the film can include lowering the temperature inside the process chamber from said first temperature to a second temperature and annealing the dielectric film to fill oxygen vacancies therein. By heating the process chamber to different temperatures, the steps of depositing and annealing the dielectric film can be accommodated in the same process chamber. More particularly, the first temperature can be in the range of 350 to 450 degrees C when depositing the dielectric film, and the second temperature can be in the range of 250 to 350 degrees C.
Furthermore, the annealing step can include providing ozone (O3) and ultraviolet light in the process chamber. Alternately, the annealing step can include providing an oxygen (O2) plasma in the process chamber, and this oxygen (O2) plasma can be generated by RF power.
According to another aspect of the present invention, a system is provided for forming a dielectric film on a microelectronic substrate. This system includes,a process chamber which receives the substrate and isolates the substrate from an external atmosphere. This system also includes means for depositing the dielectric film on the substrate in the process chamber, and means for annealing the dielectric film in the process chamber. Accordingly, the dielectric film can be deposited and annealed while reducing exposure to air thus reducing adsorption of moisture and free carbon.
In addition, the system can include a heat source which heats the substrate to a first temperature when depositing the dielectric film and which heats the substrate to a second temperature when annealing the dielectric film. In particular, the first temperature can be in the range of 350 to 450 degrees C, and the second temperature can be in the range of 250 to 350 degrees C. The heat source can be a resistive heater or a lamp. The annealing means can include an ultraviolet lamp located in the process chamber adjacent the susceptor opposite the heat source.
The methods and systems of the present invention thus allow a dielectric film to be deposited and annealed on a substrate while reducing the exposure of the dielectric film to air. Accordingly, adsorption of moisture and free carbon by the dielectric film can be reduced. The dielectric film thus formed can have improved dielectric characteristics.